A spectroscopic survey of Andromeda's Western Shelf

The Andromeda galaxy (M31) shows many tidal features in its halo, including the Giant Southern Stream (GSS) and a sharp ledge in surface density on its western side (the W Shelf). Using DEIMOS on the Keck telescope, we obtain radial velocities of M31’s giant stars along its NW minor axis, in a radial range covering the W Shelf and extending beyond its edge. In the space of velocity versus radius, the sample shows the wedge pattern expected from a radial shell, which is detected clearly here for the first tim e. This confirms predictions from an earlier model of formation of the GSS, which proposed that the W Shelf is a shell from the third orbital wrap of the same tidal debris stream that pr oduces the GSS, with the main body of the progenitor lying in the second wrap. We calculate the distortions in the shelf wedge pattern expected from its outward expansion and angular momentum, and show that these effects are echoed in the data. In addition, a hot, rela tively smooth spheroid population is clearly present. We construct a bulge-disk-halo N-body model that agrees with surface brightness and kinematic constraints, and combine it with a simulation of the GSS. From the contrasting kinematic signatures of the hot spheroid and sh elf components, we decompose the observed stellar metallicity distribution into contribut ions from each component using a nonparametric mixture model. The shelf component’s metallici ty distribution matches previous observations of the GSS superbly, further strengthening th e evidence they are connected and bolstering the case for a massive progenitor of this stream.